Apparatus and method of measuring optical properties of diffractive optical element

ABSTRACT

A laser beam emitted from a laser source is split by a beam-splitting means such as a beam sampler, and the power Q of the split beam is measured by a first detector. In addition, the power q 1  of light that has passed through a pinhole while a DOE is not set is measured by a second detector, and the power ratio α=q 1 /Q is calculated. Then, the DOE is set and the power ratio β k =q k /Q, where q k  is the power of each light beam, is calculated. The power ratio β k  is evaluated on the basis of the power ratio α, so the optical properties of a diffractive optical element, in particular, in terms of diffraction efficiency in laser-beam diffraction and intensity uniformity of split beams can be measured with high accuracy.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus and a method ofmeasuring optical properties of a Fresnel lens, a hybrid lens, adiffractive beam splitter, etc. (hereinafter referred to generically asdiffractive optical elements (DOEs)) with high accuracy, in particular,in terms of diffraction efficiency in laser-beam diffraction andintensity uniformity of split beams.

[0003] 2. Description of the Related Art

[0004] In the development of DOEs used in laser processing systems whichperform, for example, multipoint simultaneous drilling, it is necessaryto accurately evaluate optical properties such as diffraction efficiencyand intensity uniformity of split beams of the DOEs, which affect theperformance of the processing systems. A known method for evaluating theoptical properties will be described below with reference to FIG. 9.

[0005] A laser beam having a power of P_(in) is incident on a DOE 18,which is a test piece, and is split into multiple beams (seven in FIG.9). The split beams are converged on an image plane at correspondingpositions by a lens 19. The diffraction efficiency is defined as theratio of the sum of the power P_(k) of each split beam to the incidencepower P_(in). The diffraction efficiency represents the energyutilization efficiency in the use of the DOE 18.

[0006] Normally, the ratio is 0.6 to 0.9 (the diffraction efficiency is60% to 90%), and the rest 1−η indicates a loss dissipated to theenvironment as noise. Uniformity of the power P_(k) of split beams isexpressed by the standard deviation σ or the maximum-minimum range R.Equations for calculating the diffraction efficiency η, the standarddeviation σ, and the maximum-minimum range R are shown in FIG. 9. In theequations, N_(S) indicates the number of beams into which the laser beamis split (hereinafter referred to as a splitting number), and {overscore(P)}_(k) indicates the average of P_(k).

[0007] According to the above-described definitions, the diffractionefficiency and the intensity uniformity of the split beams can becalculated if the power P_(in) of the incident laser is measured with apower meter and the power P_(k) of each split beam is measured at eachfocus point where a pinhole aperture of a suitable size is set.

[0008] However, the above-described evaluating method has the followingproblems with regard to the measurement accuracy:

[0009] (a) The measurement accuracy depends on the accuracy of powermeters.

[0010] In order to measure the diffraction efficiency with highaccuracy, the power P_(in) of the incident beam and the power P_(k) ofeach split beam must be measured with high absolute accuracy. Forexample, when the splitting number of the laser beam is large, such asover a hundred, P_(k) is smaller than P_(in) by two orders of magnitudeor more. Therefore, the absolute accuracy of the power meter isextremely important. In addition, the repetition accuracy is importantfor measurement of the intensity uniformity of the split beams. However,the absolute accuracy of commercial power meters is normally ±3% to ±5%,and is not sufficient.

[0011] (b) Power stability of the laser beam greatly affects themeasurement accuracy.

[0012] When the laser power is unstable, measurement values of P_(in)and P_(k) vary and the measurement accuracy decreases. The powerstability of commercial carbon dioxide lasers is normally ±5% to ±10%,and is also not sufficient.

[0013] (c) The size of the pinhole for allowing the split beams to passtherethrough affects the measurement results.

[0014] In order to accurately measure the power P_(k) of each splitbeam, the pinhole size must be optimized. However, since the intensitydistribution is widely spread with very low intensity side lobes at eachspot, it is extremely difficult to determine the pinhole size. If thepinhole size is too small, the power cannot be sufficiently collectedand P_(k) will be smaller than the actual value, and if the pinhole sizeis too large, environmental noise and the power of the neighboring splitbeams will be collected and P_(k) will be larger than the actual value.Thus, the measurement results vary in accordance with the pinhole size,and sufficient reliability cannot be obtained.

[0015] (d) The quality of the laser beam greatly affects the measurementaccuracy.

[0016] If the size of the focal spots increases because of thetransverse mode characteristics and the wavefront aberration of thelaser beam and the shape of the focal spots is distorted, it becomesincreasingly difficult to determine the pinhole size.

[0017] (e) Characteristics of a lens included in the measurement systemgreatly affect the measurement accuracy.

[0018] The power P_(k) of each split beam is affected by thetransmittance of a lens used. When the transmittance decreases, themeasured diffraction efficiency decreases accordingly. In addition, theaberration of the lens also distorts the focal spots similarly as thelaser quality does. Since off-axis aberrations depend on the incidenceangle of the beam onto the lens, the value of P_(k) decreases as thesplitting number increases and the incidence angle increasesaccordingly.

SUMMARY OF THE INVENTION

[0019] An object of the present invention is to provide an apparatus formeasuring optical properties of a DOE in which the above-describedproblems can be solved to achieve a high-accuracy measurement.

[0020] According to the present invention, an apparatus for measuringoptical properties of a DOE includes a laser source which emits a laserbeam; a beam-splitting means of splitting the laser beam into a mainbeam and a reference beam; a first measuring means of measuring thepower or the energy of the reference beam; a DOE (test piece) whichsplits the main beam into a plurality of split beams; a mask whichallows one of the split beams to pass therethrough; a mask-moving meansof moving the mask along at least two axial directions in a planeperpendicular to an optical axis; a second measuring means of measuringthe power or the energy of the split beam which passes through the mask;and a calculating means of calculating the ratio of the power or theenergy measured by the first measuring means to the power or the energymeasured by the second measuring means.

[0021] The measurement apparatus may further include a converging lenswhich is disposed in the rear of the DOE (test piece). According to theDOE, the DOE (test piece) may be placed at the front focal point of theconverging lens. The converging lens may be, for example, an fsinθ lensor a single lens.

[0022] According to the present invention, optical properties of the DOE(test piece) are measured by using the above-described apparatus by thefollowing two steps:

[0023] First step: A power ratio α=q₁/Q is calculated without disposingthe DOE. Q is the power of the incident laser beam and measured by thefirst measuring means, and q₁ is the power of the light that has passedthrough the mask and it is measured by the second measuring means.

[0024] Second step: The DOE is set, and a power ratio β_(k)=q_(k)/Q beamis calculated for each split beam. q_(k) is the power of each of thesplit beams which are split by the DOE.

[0025] The intensity of each split beam, the diffraction efficiency ofthe DOE, and the intensity uniformity of split beams are calculated onthe basis of the power ratios α and β_(k).

[0026] In the first and second steps, the ratio of the energy of theincident laser beam to that of the light that has passed through themask may be obtained by measuring them instead of calculating the powerratio.

[0027] The converging lens may be unnecessary depending on the DOE (testpiece). In the case in which the converging lens is not used, areference lens having the same focal length as that of the DOE is set inplace of the DOE, and the power or the energy of light split by thereference lens may be measured by the second measuring means.

[0028] Alternatively, the optical properties of the DOE (test piece) mayalso be measured by the following steps:

[0029] First step: A reflective mirror is disposed in front of theconverging lens in place of the test piece. Then, while changing theincidence angle θ of the laser beam incident on the converging lens withthe reflective mirror, the dependency α(θ)=q₁/Q of the power ratio onthe incidence angle 0 of the laser beam is measured, where Q is thepower measured by the first measuring means and q₁ is the power measuredby the second measuring means, and a correction factor γ(θ)=α(θ)/α(θ) isobtained by normalizing the measured dependency α(θ) with α(0);

[0030] Second step: The power ratio α=q₁/Q in the state in which thetest piece is not set or a reference lens having the same focal lengthas that of the test piece is set in place of the test piece iscalculated.

[0031] Third step: The test piece is set and a power ratio β_(k)=q_(k)/Qis measured for each split beam in the state in which the test piece isset, where q_(k) is the power of each split beam measured by the secondmeasuring means. Then, the power ratio β_(k) is divided by thecorrection factor γ(θ_(k)) corresponding to the diffraction angle θ_(k)of each split beam to obtain a corrected power ratioβ_(k)′=β_(k)/γ(θ_(k)).

[0032] The intensity of each split beam, the diffraction efficiency ofthe DOE, and the intensity uniformity of split beams are calculated onthe basis of the power ratios α and β_(k)′.

[0033] As described above, according to the measurement apparatus andthe measurement method of the present invention, the power ratio α,which is intrinsic to the measurement system, is determined as areference and the power ratio β_(k) or β_(k)′ in the state where the DOEis provided is evaluated on the basis of the power ratio α. Accordingly,the most important optical properties of the DOE (test piece), i.e. thediffraction efficiency and the intensity uniformity of the split beams,and the positional accuracy of the focal points, can be measured withhigh accuracy, because the measurement accuracy of the method of thepresent invention is hardly affected by the accuracies of the powermeters, the laser stability, the pinhole size, the quality of the laser,and characteristics of the lens used in the measurement system.

[0034] In the case in which the converging lens is provided, the opticalproperties of DOEs which cannot converge beams to a suitable size at asuitable position can also be evaluated.

[0035] In addition, in a apparatus where the DOE is placed at the frontfocal point of the converging lens, each split beam from the DOE, evenwhich have a large diffraction angle, is converged by the lens andirradiates the pinhole vertically, so the power of each split beam canbe measured under more stable conditions, whereby the measurementaccuracy increases.

[0036] In addition, when the fsinθ lens is used as the converging lens,errors due to the lens aberrations are reduced and the positionalaccuracy of the spots can be improved, and therefore the measurementaccuracy can be further improved.

[0037] Furthermore, even when the characteristics of the converging lensare dependent on the incidence angle, the influence of the incidenceangle on the measurement values can be eliminated through the correctionprocess, and high-accuracy measurement can also be performed in such acase.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a diagram showing the measurement principle of anapparatus and a method according to an embodiment of the presentinvention;

[0039]FIG. 2 is a diagram showing the overall construction of themeasurement apparatus;

[0040]FIG. 3 is a diagram showing the verification result of thepower-ratio measurement accuracy;

[0041]FIG. 4 is a diagram showing the measurement result and thesimulation analysis result of a DOE whose splitting number is seven;

[0042]FIG. 5 is a diagram showing the examination result of a dependencyof split-beam intensity on a pinhole size;

[0043]FIG. 6 is a diagram showing the construction of a hybrid lens;

[0044]FIG. 7 is a diagram showing the manner in which a Fresnel DOEsplits a laser beam and the split beams converge;

[0045]FIG. 8 is a diagram showing the manner in which optical propertiesof the Fresnel DOE are measured;

[0046]FIG. 9 is a diagram showing a known evaluation method; and

[0047]FIG. 10 is a diagram showing the manner in which opticalproperties of the hybrid lens are measured.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0048]FIG. 1 is a diagram showing the measurement principle of a methodaccording to an embodiment of the present invention. In the case inwhich a DOE (test piece) 18 cannot converge beams to a suitable size ata suitable position, for example, when the DOE 18 is a Fourier DOE(whose focal length is infinitely large), a Fresnel DOE having anegative focal length, or a Fresnel DOE having a positive, but longfocal length, a converging lens 9 is necessary. However, when the DOE 18is a Fresnel DOE having a suitable positive focal length, the converginglens 9 may be omitted. Although the lens 9 shown in the figure is anfsinθ lens including a plurality of lens elements, a single lens elementmay also be used.

[0049] As shown in the figure, an apparatus is designed such that a partof a laser beam emitted from a laser source is separated by abeam-splitting means 5, such as a beam splitter or a beam sampler, andthe power Q thereof is measured by a first detector 6. The apparatus isarranged such that a light beam incident on the DOE 18 is split intomultiple beams, and the power q_(k) of each split beam is measured bythe power meter of a second detector 12, through a pinhole (whosediameter is, for example, 50 to 200 μmφ) formed in a mask 10.

[0050] The mask 10 can move together with a water-cooled damper 11 in atleast two axial directions in a plane perpendicular to an optical axis,so that the power q_(k) of each split beam can be detected individuallyby the power meter of the second detector 12.

[0051] As a first step, the measurement is performed without the DOE 18,and the ratio α between the power Q measured by the first detector 6 andthe power q₁ of light having passed through the pinhole in the mask 10and measured by the second detector 12, is calculated as α=q₁/Q.

[0052] Then, as a second step, the DOE 18 is set, and the ratio of thepower q_(k) of each split beam to the power Q is calculated asβ_(k)=q_(k)/Q

[0053] When the converging lens 9 is used, the DOE 18 is preferablyplaced at the front focal point of the converging lens 9. In such acase, the system is telecentric on the image side, and the power of eachsplit beam can be measured under stable conditions, since the convergedbeams enter the pinhole vertically even when the split beams having alarge diffraction angle are incident on the converging lens 9 from theDOE 18.

[0054] When the power ratios α and β_(k) are calculated, the intensityof each split beam and the diffraction efficiency and the intensityuniformity of the split beams of the DOE 18 can be obtained as follows:$\begin{matrix}{{P_{k} = {\beta_{k}/\alpha}}} & (1) \\{{\eta = {\sum\limits_{k}{\beta_{k}/\alpha}}}} & (2) \\{{\delta = \sqrt{\sum\limits_{k}{{\left( {\beta_{k} - {\overset{\_}{\beta}}_{k}} \right)^{2}/N_{S}}\alpha^{2}}}}} & (3) \\{{R = {\left\lbrack {{\max \left( \beta_{k} \right)} - {\min \left( \beta_{k} \right)}} \right\rbrack \sqrt{\beta_{k}}}}} & (4)\end{matrix}$

[0055] The above-described measurement method is characterized in thatthe reference power ratio α is determined at the first step. Althoughthe value of the power ratio a itself has no meaning, it can beconsidered as an intrinsic value of the measurement system representingthe characteristics of all the components including the laser, the beamsplitter, the lenses, the pinhole, and the detectors, except for the DOE18. More specifically, the power ratio β_(k) obtained when the DOE 18 isset, is evaluated on the basis of the power ratio α, which is intrinsicto the measurement system, so that error factors can be eliminated tothe extent possible and the properties of the DOE 18 alone can beevaluated.

[0056] When the above-described measurement method is applied, theproblems described in items (a) to (e) can be solved, or alleviated, asfollows:

[0057] (a) Accuracy of the power meters: Since the power ratio ismeasured, it is not necessary that the absolute accuracy of the powermeters be high (the repetition accuracy and linearity affect themeasurement accuracy). When the splitting number is large, such as overa hundred, the range of power which must be measured by the detectorscan be reduced by setting the power of the laser apparatus to a lowlevel in the first step and to a high level in the second step.Accordingly, the linearity error of the power meters would not be amajor problem.

[0058] (b) Stability of the laser power: Even when the laser power isunstable, the measured power ratio does not change theoretically, so themeasurement accuracy does not decrease. Only in the case that the laserpower varies faster than the speed of response of the power meter, themeasurement accuracy presumably decreases.

[0059] (c) Pinhole size: Even when the pinhole size varies, the powerratios α and β_(k) increases/decreases in a constant proportion, themeasurement result is minimally affected. The pinhole size can beincreased in a range such that noise and the neighboring split beams donot enter therein, and alternatively the size can be reduced to smallerthan the focal spot size.

[0060] (d) Quality of the laser: Even when the size and the shape of thefocal spots are different from those expected due to the laser mode andthe wavefront aberration, it minimally affects the measurement resultbecause of the above-described reasons.

[0061] (e) Lens characteristics: Even when the transmittance of the lensused in the measurement system is low, its effects are included in bothof the power ratios α and β_(k), so the measurement result is notaffected. Although the distortion of the focal spots due to on-axisaberrations is also not a problem, a countermeasure is necessary ifthere are off-axis aberrations, that is, dependency on the incidenceangle. In such a case, the dependency on the incidence angle is measuredin advance, and β_(k) is corrected by using the measured value as acorrection value. More specifically, a reflective mirror is placed infront of the converging lens in place of the DOE, and the dependencyα(θ)=q₁/Q of the power ratio on the incidence angle is measured whilechanging the angle θ of the laser beam which is incident on theconverging lens by using the reflective mirror. The dependency of thepower ratio on the incidence angle is normalized by using the powerratio α(0) corresponding to θ=0° to obtain the correction factorγ(θ)=α(θ)/α(0). Then, the power ratio of each split beam β_(k)′=q_(k)/Qobtained when the DOE is set, is divided by the correction factorγ(θ_(k)) corresponding to the diffraction angle θ_(k) of each splitbeam, so the corrected power ratio β_(k)′=β_(k)/γ(θ_(k)) is obtained.

[0062] A similar correction is also possible in the case in which thetransmittance depends on the incidence angle. When the fsinθ lensdesigned and manufactured for large incidence angles is used, theaberrations for beams having large diffraction angles are alsocorrected, so that the problem due to the aberrations does not occur.Such an fsinθ lens can improve the positional accuracy of the focalspots, and is an ideal lens.

[0063]FIG. 2 is a diagram showing the overall construction of themeasurement apparatus. With reference to the figure, the measurementapparatus includes a CO₂ laser source 1 including a power source, ashutter 2 for switching the guide light, a control panel 3 for the CO₂laser source 1 and the shutter 2, a mirror box 4 for changing thedirection of the laser beam, the beam-splitting means (beam sampler) 5,the first detector 6, a zoom expander 7, two bent mirrors 8, the DOE 18,the converging lens 9 (fsinθ lens in the figure), the mask 10 with thepinhole, the water-cooled damper 11 on which the mask 10 is mounted, thesecond detector 12, a three-axis automatic stage 13 for moving thesecond detector 12, the damper 11, and the mask 10, a controller 14 forthe three-axis automatic-control slide stage 13, an indicator 15 of themeasured power, and a personal computer 17 with an interface board 16.

[0064] In FIG. 2, the laser beam is emitted from the CO₂ laser source 1,enters a test-piece chamber 30, and is split into a main beam and areference beam by the beam sampler 5. The power of the reference beam ismeasured by the first detector 6. The main beam passes through the zoomexpander 7 where the diameter of the main beam is increased, isreflected by the bent mirrors 8, and enters the DOE 18. Then, the splitbeams are converged by the converging lens 9 (this may be a singlelens). The mask 10 is moved by the three-axis automatic stage 13, andthe power of each split beam that has passed through the pinhole in themask 10 is measured by the second detector 12. The automatic stage 13and the power meter of the second detector 12 are controlled by thepersonal computer 17, and the entire measurement process including stagemovement and the power measurement can be performed automatically. Inthe automatic measurement, the automatic adjustment of the pinhole isalso performed, as described below.

[0065] In the automatic measurement, when the coordinates of the focalpoints of the split beams on the image plane to be measured aredetermined, the pinhole is moved vertically and horizontally at apredetermined pitch (for example, 10 μm) in a predetermined areacentered on the determined coordinates while measuring the power ratioat each position. Then, at the position where the maximum power ratio isobtained, the mask is moved along the optical axis and the coordinatealong the optical axis where the maximum power ratio is obtained, isdetermined. The result is output in a list. This process isautomatically repeated at each of the focal points of the split beams.

[0066]FIG. 3 shows the results of a measurement of the power ratio αthat was performed without the DOE to verify the accuracy of theabove-described measurement apparatus. The power of the laser apparatuswas varied in a range such that the power value of the second detector12 varied in the range of 0.1 W to 20 W, and α was measured ten timesfor each power value. The average value of α is plotted in FIG. 3. Twotypes of detectors were prepared as the second detector 12: Type A whosemeasurement range was up to 30 W, and Type B whose measurement range wasup to 2 W. Since the laser output was varied in a wide range fromseveral hundred mW to over 20 W, the degree of power variation was largeand the measurement values of the first and second detectors were varied±10% to ±30%. However, as shown in FIG. 3, for both detectors of Type Aand Type B, the measured power ratio a was nearly constant for all thepower values. The average of all α which was measured ten times for eachpower value and the deviation (2σ) thereof were α=191.7±3.7% for thedetector of Type A and α=125.8±3.1% for the detector of Type B.Accordingly, the target accuracy of less than ±5% was achieved. Thereason why the value of α obtained by the detector of Type A and thatobtained by the detector of Type B are different from each other isbecause these detectors have different structures (the size of thelight-receiving surface, the distance from the pinhole, etc.). From thisresult, it has been confirmed that the measurement accuracy of theabove-described measurement apparatus is sufficiently high.

[0067] Next, the result of a measurement of the diffraction efficiencyand the intensity uniformity of the split beams of the DOE performed byusing the measurement apparatus shown in FIG. 2 will be described below.

[0068] First, the measurement result of a binary phase DOE whosesplitting number is seven will be described. In this case, a meniscusaspherical lens whose focal length was 5 inches (127 mm) and diameterwas 2 inches (50.8 mmφ) was used. The pitch of the focal points was 0.5mm. Other conditions were such that the expander magnification was 2,the pinhole diameter was 280 μmmφ, and the detector of Type A was usedas the second detector 12. FIG. 4 shows the result of five measurementsand the results of analysis (the simulation result of the influence ofline width error and etching depth error occurred in themicrofabrication of the DOE). The intensities of seven split beams areplotted in a bar graph shown in FIG. 4. As is apparent from FIG. 4, thevalues obtained by the five measurements are almost constant, and thedistribution pattern of the measured values is similar to that of theanalysis values. In particular, as has been predicted by the simulation,the fact that the intensity of the zero-order split beam at the centeris lower than the intensities of other split beams due to the line widtherror can be confirmed by the measurement results. The analysis value ofthe diffraction efficiency was 74.3%, and the measurement result thereofwas 72.7%±0.3%. In addition, the analysis value of the intensityuniformity (σ) of the split beams was 1.7% and the measurement resultthereof was 3.0%, which was relatively larger than the analysis value.

[0069] In addition, measurements for a sixteen-level phase DOE whosesplitting number was 7×7=49 were also performed under the sameconditions as the above-described case where the DOE whose splittingnumber was seven, was used. Also in this case, the distribution patternof the intensities of the split beams obtained by the analysis result(where line width error and etching depth error were taken into account)and that obtained by the measurement results were extremely similar toeach other. The analysis value of the diffraction efficiency was 88.5%,and the measurement result thereof was 87.8%. In addition, the analysisvalue of the intensity uniformity (σ) of the split beams was 5.3% andthe measurement result thereof was 8.0%.

[0070] Next, the dependency of the intensities of the split beams on thepinhole size was investigated. FIG. 5 shows the measurement results ofthe intensities of the split beams when the diameter of the pinhole wasset to 60 μm, 100 μm, 140 μm, 180 μm, and 280 μm. As is apparent fromFIG. 5, the measurement results did not change even when the pinholesize was changed.

[0071]FIG. 6 shows a hybrid lens 22 which has an aspheric surface 20 anda brazed surface-relief microstructure 21 on the aspheric surface, andthey realize functions of refraction and diffraction, respectively. Thefocal length of the hybrid lens is 127 mm, and the diameter thereof is50.8 mm. In addition, the hybrid lens is composed of zinc selenide(ZnSe). The diffraction efficiency of this hybrid lens was measured bythe following steps:

[0072] First step: An aspheric lens 26 (reference lens) having the samefocal length (127 mm) as the hybrid lens (DOE) was set in themeasurement apparatus as shown in FIG. 10, and the reference power ratioα=q₁/Q was determined.

[0073] Second step: The hybrid lens 27 was set in place of the asphericlens 26 and the power ratio α==q₁/Q was determined.

[0074] As described above, the converging lens 9 is not used in thiscase. In addition, the hybrid lens 27 used in this case is a converginglens and does not have the beam-splitting function, so while the powerratio determined at the second step is only β₁. Accordingly, althoughthe main target of the measurement apparatus and the measurement methodis beam-splitting DOEs, it can of course be used for measuring opticalproperties of light-converging DOEs having no beam-splitting function(Fresnel lenses, hybrid lenses, etc.).

[0075] The diffraction efficiency η=β₁/α was calculated from the resultsobtained at the first and the second steps; η=97.3% was obtained forSample 1 and η=98.6% for Sample 2. The diffraction efficiency of aFresnel lens (where a surface-relief microstructure is formed on a flatsurface) may also be obtained in a similar manner.

[0076] The optical properties of a Fresnel DOE were measured by usingthe measurement apparatus shown in FIG. 2. As shown in FIG. 7, theFresnel DOE 23 has both the beam-splitting function and thelight-converging function. The focal length of the Fresnel DOE 23 was254 mm, the number of focal spots was 7×7=49, and the spot pitch was 1mm. In addition, the Fresnel DOE 23 was made of ZnSe.

[0077] As shown in FIG. 8, the fsinθ lens 9 in the measurement apparatusshown in FIG. 2 was replaced by a meniscus aspherical lens 19, and areference lens 24 or the Fresnel DOE 23 was placed at the front focalpoint of the lens 19. The measurement was performed by the followingsteps:

[0078] First step: A plano-convex lens (reference lens 24) made of ZnSehaving the same focal length (254 mm) as the Fresnel DOE 23 was set andthe reference power ratio α=q₁/Q was determined.

[0079] Second step: The Fresnel DOE 23 was set in place of theplano-convex lens and the power ratio β_(k)=q_(k)/Q of each split beamwas determined.

[0080] Then, the diffraction efficiency η and the intensity uniformity σof the split beams were calculated from Eq. (1) to (4). As a result,η=71.7% and σ=3.9% were obtained for Sample 1 and η=72.1% and σ=3.7%were obtained for Sample 2. Since the Fresnel DOE 23 also have thelight-converging function, it is possible to thereby perform measurementwithout a converging lens as shown in FIG. 10.

[0081] Although in the above described examples the optical propertiesof DOEs for CO₂ lasers are determined by using a CO₂ laser, a similarmeasurement apparatus may also be constructed for DOEs for other lasers(YAG lasers, etc.) using the corresponding laser and optical componentstherefor (mirrors, converging lenses, etc.), and the optical propertiescan be measured by a similar method. In such a case, the effects of thepresent invention also can be obtained.

What is claimed is:
 1. An apparatus for measuring optical properties ofa diffractive optical element, comprising: a laser source which emits alaser beam; beam-splitting means of splitting the laser beam into a mainbeam and a reference beam; first measuring means of measuring the poweror the energy of the reference beam; a test piece which splits the mainbeam into a plurality of split beams; a mask which allows one of thesplit beams to pass therethrough; mask-moving means of moving the maskalong at least two axial directions in a plane perpendicular to anoptical axis; second measuring means of measuring the power or theenergy of the split beam that has passed through the mask; andcalculating means of calculating the ratio of the power or the energymeasured by the first measuring means to the power or the energymeasured by the second measuring means.
 2. An apparatus for measuringoptical properties of a diffractive optical element according to claim1, further comprising a converging lens which is disposed in the rear ofthe test piece.
 3. An apparatus for measuring optical properties of adiffractive optical element according to claim 2, wherein the test pieceis placed at the front focal point of the converging lens.
 4. Anapparatus for measuring optical properties of a diffractive opticalelement according to claim 2, wherein the converging lens is an fsinθlens.
 5. An apparatus for measuring optical properties of a diffractiveoptical element according to claim 3, wherein the converging lens is anfsinθ lens.
 6. A method of measuring optical properties of a diffractiveoptical element by using the apparatus according to claim 1, comprisingthe steps of: calculating a power ratio α=q₁/Q, where Q is the powermeasured by the first measuring means and q₁ is the power measured bythe second measuring means, in the state in which a reference lenshaving the same focal length as that of the test piece is set in placeof the test piece; and setting the test piece and calculating a powerratio β_(k)=q_(k)/Q for each split beam, where q_(k) is the power ofeach split beam measured by the second measuring means, in the state inwhich the test piece is set.
 7. A method of measuring optical propertiesof a diffractive optical element by using the apparatus according toclaim 2, comprising the steps of: calculating a power ratio α=q₁/Q,where Q is the power measured by the first measuring means and q₁ is thepower measured by the second measuring means, in the state in which thetest piece is not set or a reference lens having the same focal lengthas that of the test piece is set in place of the test piece; and settingthe test piece and calculating a power ratio β_(k)=q_(k)/Q for eachsplit beam, where q_(k) is the power of each split beam measured by thesecond measuring means, in the state in which the test piece is set. 8.A method of measuring optical properties of a diffractive opticalelement by using the apparatus according to claim 3, comprising thesteps of: calculating a power ratio α=q₁/Q, where Q is the powermeasured by the first measuring means and q₁ is the power measured bythe second measuring means, in the state in which the test piece is notset or a reference lens having the same focal length as that of the testpiece is set in place of the test piece; and setting the test piece andcalculating a power ratio β_(k)=q_(k)/Q for each split beam, where q_(k)is the power of each split beam measured by the second measuring means,in the state in which the test piece is set.
 9. A method of measuringoptical properties of a diffractive optical element by using theapparatus according to claim 2, comprising the steps of: disposing areflective mirror in front of the converging lens in place of the testpiece, determining the dependency α(θ)=q₁/Q of the power ratio on anincidence angle θ of the laser beam incident on the converging lens,where Q is the power measured by the first measuring means and q₁ is thepower measured by the second measuring means, while changing theincidence angle θ with the reflective mirror, and normalizing thedetermined dependency by α(0) to obtain a correction factorγ(θ)=α(θ)/α(0); calculating a power ratio α=q₁/Q in the state in whichthe test piece is not set or a reference lens having the same focallength as that of the test piece is set in place of the test piece; andsetting the test piece, calculating a power ratio β_(k)=q_(k)/Q for eachsplit beam, where q_(k) is the power of each split beam measured by thesecond measuring means, in the state in which the test piece is set, anddividing the power ratio β_(k) by the correction factor y(Ok)corresponding to the diffraction angle θ_(k) of each split beam toobtain a corrected power ratio β_(k)′=β_(k)/γ(θ_(k)).
 10. A method ofmeasuring optical properties of a diffractive optical element by usingthe apparatus according to claim 3, comprising the steps of: disposing areflective mirror in front of the converging lens in place of the testpiece, determining the dependency α(θ)=q₁/Q of the power ratio on anincidence angle θ of the laser beam incident on the converging lens,where Q is the power measured by the first measuring means and q₁ is thepower measured by the second measuring means, while changing theincidence angle θ with the reflective mirror, and normalizing thedetermined dependency by α(0) to obtain a correction factorγ(θ)=α(θ)/α(0); calculating a power ratio α=q₁/Q in the state in whichthe test piece is not set or a reference lens having the same focallength as that of the test piece is set in place of the test piece; andsetting the test piece, calculating a power ratio β_(k)=q_(k)/Q for eachsplit beam, where q_(k) is the power of each split beam measured by thesecond measuring means, in the state in which the test piece is set, anddividing the power ratio β_(k) by the correction factor γ(θ_(k))corresponding to the diffraction angle θ_(k) of each split beam toobtain a corrected power ratio β_(k)′=β_(k)/γ(θ_(k)).